Concentrated and dilute acids
How does dilute strong acid compare with a concentrated weak acid?
It depends on the pKa of the weak acid and the concentrations of both acids (calculation shown below). Firstly let's discuss it a bit.
Typical concentrations of commercially available concentrated acids:
Acid |
Density |
Molarity (M) |
Acetic acid 99.5% |
1.05 |
17.4 |
Hydrochloric acid 36% |
1.18 |
11.65 |
Nitric acid 70% |
1.42 |
15.8 |
Perchloric acid 70% |
1.67 |
11.6 |
Sulfuric acid 98% |
1.84 |
18.4 |
Acetic acid is the only weak acid on the list. Dilute acetic acid is vinegar; yes, the same you put in your chips. But it can also be bought in a very concentrated form (99.5%), so that it contains only 0.5% water, and it is called glacial acetic acid. This one you certainly don't want in your chips, and you won't want to be near an open bottle. Silicone tubes for DIY work usually come with acetic acid, that although not concentrated, can cause irritations in the respiratory tract and in the eyes.
Sulphuric acid can also be bought in a very concentrated form (98%) and it has inumerous applications in various industries. One important application is as a dehydrating agent.
Interestingly, hydrochloric acid (HCl) is not sold in a high concentration, but 36% at the most... That is because it fumes a lot and it is difficult to handle at higher concentrations; its fumes are very nasty.
Nitric acid is available at a 70% concentration. In concentrations of 86% or above it is know as fuming nitric acid. It can be available in a 100% concentration for rocket fuel applications, as it is a strong oxidiser.
Perchloric acid is stronger than sulphuric and nitric acids and it is a potent oxidiser. It is safe to be used at 70% concentration but above that it would immediately burn (or even explode) any organic material that gets in contact.
Calculation: dilute strong acid versus concentrated weak acid
Let's suppose that we have 17 M acetic acid (concentrated weak acid) and 0.001 M HCl (dilute strong acid). To compare the strength of either solution, we need to calculate the concentration of H+ , [ H+ ], in each case.
(pH's are calculated as shown here)
-Acetic acid 17 M:
Ka = 1.8 * 10 -5
[H3O+] * [H3O+] = 17 * 1.8 * 10 -5 --> [H3O+] = 5.5 * 10 -3
- HCl 0.01 M
This is a strong acid, so that it is completely ionised.
That means that [H3O+] = 0.001 M = 10 -3
Conclusion: In this case, the concentrated acetic acid (weak acid) solution is more acidic than the dilute HCl (strong acid) solution.